These studies are primarily directed towards understanding the expression of endogenous murine leukemia viral (MuLV) sequences in mice and their role in the generation of exogenous leukemogenic mink cell focus-forming (MCF) MuLVs. Endogenous MuLV DNA segments were molecularly cloned from the genome of BALB/c and AKR/J mice and extensively characterized by nucleic acid hybridization, nucleotide sequencing, and in DNA transfection experiments. The results are summarized as follows. 1) The long terminal repeats (LTRs) associated with a majority of the endogenous MuLV DNAs are distinct from the LTRs present in known exogenous MuLVs due to the presence of 190 bp segment which possesses transposon-like features. This insert is located 48 bp upstream from the CCAAT box in the endogenous LTRs. 2) Nucleotide sequence comparison in the env region between an endogenous clone (A-12) and exogenous MCF247 MuLV DNA demonstrated that the two sequences were identical except for a few base changes. Comparison between the MCF and xenotropic MuLV sequences allowed identification of MCF unique nucleotides in env based upon which a 16 bp MCF-specific DNA probe was synthesized which did not react with xenotropic or ecotropic proviruses. 3) Recombinant MuLVs containing 3 feet env sequences from two different endogenous MuLV DNA clones encoded distinct p15E envelope protein products. The LTR associated with an endogenous African green monkey (AGM) retrovirus clone was also characterized. Nucleotide sequencing analysis indicated conservation of CCAAT and TATA regulatory signals and very little homology to the baboon endogenous viral LTR. The 3 feet AGM LTR demonstrated enhancer activity in an assay in which chloramphenicol acetyl transferase (CAT) gene expression was monitored.